Battery Capacity Calculator - Amp-Hours & Watt-Hours
Calculate battery capacity in amp-hours (Ah) and watt-hours (Wh) from current draw and runtime. Free battery sizing tool for devices, UPS, solar systems, drones, and electronics projects.
About This Calculator
The Battery Capacity Calculator - Amp-Hours & Watt-Hours helps you calculate the required battery capacity in amp-hours (Ah) and energy in watt-hours (Wh) based on your device's current draw and runtime. It is essential for sizing batteries for drones, medical devices, UPS systems, solar power setups, IoT gadgets, cameras, and portable electronics. Engineers, hobbyists, and DIY enthusiasts use this tool to select the right battery for any application.
The calculation uses the standard electrical formula: Capacity (Ah) = Current (A) × Time (h), and Energy (Wh) = Capacity (Ah) × Voltage (V). For example, a device drawing 5A for 10 hours at 12V needs a 50Ah battery storing 600Wh of energy. The calculator also provides a detailed breakdown showing each step of the computation. Always add a 20% safety margin for lead-acid batteries to account for depth-of-discharge limits.
Regional Notes
India (IN): Battery capacities for inverters and home UPS systems are typically rated at 12V or 24V. Common sizes range from 100Ah to 200Ah for home backup. Use 60-80% depth of discharge for lead-acid tubular batteries.
United States (US): Deep-cycle marine and RV batteries are commonly rated at 12V in 50Ah to 200Ah sizes. Solar installations use 24V or 48V battery banks. Lithium-ion batteries allow 90-100% depth of discharge.
United Kingdom (UK): Caravan and motorhome batteries typically range from 75Ah to 110Ah at 12V. Off-grid solar systems commonly use 24V or 48V lithium battery banks with usable capacity calculated at 80-90% DoD.
Frequently Asked Questions
How is battery capacity calculated?
Battery capacity in amp-hours (Ah) is calculated by multiplying the current draw in amperes (A) by the runtime in hours (h): Ah = A × h. For example, a device drawing 5A for 10 hours needs a 50Ah battery. Energy in watt-hours is capacity times voltage: Wh = Ah × V.
What is the difference between Ah and Wh?
Amp-hours (Ah) measure charge capacity, while watt-hours (Wh) measure energy. Wh = Ah × V. A 50Ah battery at 12V stores 600Wh, giving a better comparison across different voltage systems.
How do you size a battery for a UPS?
Calculate total load in watts, multiply by desired backup time in hours, then divide by battery voltage and add a 20% safety margin. For a 500W load needing 2 hours at 12V: (500 × 2) / 12 × 1.2 = 100Ah.
What battery capacity do I need for solar?
Calculate your daily energy usage in watt-hours, divide by battery voltage, then multiply by days of autonomy (typically 1-3). A home using 5kWh per day at 12V needs approximately 417Ah for one day of backup, or about 200Ah at 24V.
How long will a 100Ah battery last?
A 100Ah battery at 12V provides 1200Wh. Runtime = Capacity / Current draw. At 10A you get 10 hours, at 5A you get 20 hours. For lead-acid batteries, derate by 20% to avoid deep discharge damage, giving 8 hours at 10A.
What is C-rate in batteries?
C-rate describes discharge rate relative to battery capacity. A 1C rate discharges fully in 1 hour, 0.5C in 2 hours. A 100Ah battery at 0.5C delivers 50A for 2 hours. Higher C-rates reduce usable capacity due to internal resistance losses.
How does temperature affect battery capacity?
Battery capacity decreases significantly in cold conditions. Lead-acid batteries lose 20-50% of capacity below freezing. Lithium-ion batteries maintain better cold performance but degrade faster above 40°C. The optimal operating temperature is 20-25°C for all battery types.
Can I use this calculator for lithium and lead-acid batteries?
Yes, the calculator works for any battery chemistry. However, lead-acid batteries should only be discharged to 50% depth of discharge (DoD), so multiply the calculated capacity by 2 for lead-acid. Lithium-ion batteries can safely use 80-100% DoD, so the calculated capacity is closer to the actual battery size needed.